title: Cell signalling and the hormonal stimulation of the hepatic glycine cleavage enzyme system by glucagon.
abstract: The glycine cleavage enzyme system (GCS) is found in mitochondria. In liver it is activated by glucagon and other hormones but it is not known how the hormonal signal is transmitted to the mitochondria. We found that the cell-permeant protein phosphatase inhibitor okadaic acid stimulated flux through GCS and could induce a significant increase in the sensitivity of GCS and of glycogenolysis to glucagon. Half-maximal stimulation of GCS by glucagon occurred at 3.2+/-0.6 nM, whereas it was fully activated at 0.3 nM in the presence of 1 microM okadaic acid. The protein kinase A agonist adenosine-3',5'-cyclic monophosphorothioate, Sp isomer (10 microM) stimulated the GCS flux by approx. 100%. This stimulation was inhibited by the protein kinase A antagonist 8-bromoadenosine-3', 5'-cyclic monophosphorothioate, Rp isomer (Rp-8-Br-cAMPS). Although Rp-8-Br-cAMPS significantly inhibited glucagon-stimulated glycogenolysis it had no effect on the glucagon-stimulated GCS flux. These results indicate that a cytoplasmic phosphorylated protein is involved in transmitting glucagon's effect to the mitochondria. However, protein kinase A does not have a necessary role in transmitting glucagon's signal. We also examined the role of protein kinase C because angiotensin II also stimulated flux through GCS. However, the phorbol ester PMA had no effect on either GCS or on glycogenolysis.
authors.affiliation: Department of Biochemistry, Memorial University of Newfoundland, St. John's, Newfoundland, Canada A1B 3X9.
authors.forename: G M
authors.initials: GM
authors.lastname: Mabrouk
authors.forename: M
authors.initials: M
authors.lastname: Jois
authors.forename: J T
authors.initials: JT
authors.lastname: Brosnan
mesh_terms: D000594:Amino Acid Oxidoreductases / Q000378:metabolism*; D000804:Angiotensin II / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism; D002352:Carrier Proteins / Q000378:metabolism*; D000242:Cyclic AMP / Q000378:metabolism; D004789:Enzyme Activation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005934:Glucagon / Q000502:physiology*; D005998:Glycine / Q000378:metabolism*; D008297:Male; D008930:Mitochondria, Liver / Q000201:enzymology*; D009097:Multienzyme Complexes / Q000378:metabolism*; D019319:Okadaic Acid / Q000494:pharmacology; D011493:Protein Kinase C / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D018027:Receptors, Glucagon / Q000378:metabolism; D015398:Signal Transduction; D019284:Thapsigargin / Q000494:pharmacology; D014166:Transferases / Q000378:metabolism*
chemical_list: D002352:Carrier Proteins; D004791:Enzyme Inhibitors; D009097:Multienzyme Complexes; D018027:Receptors, Glucagon; C064525:glycine cleavage system; D000804:Angiotensin II; D019319:Okadaic Acid; D019284:Thapsigargin; D005934:Glucagon; D000242:Cyclic AMP; D000594:Amino Acid Oxidoreductases; D014166:Transferases; D011493:Protein Kinase C; D002118:Calcium; D005998:Glycine